Tunicates: The Filter-Feeding Chordates Shaping Marine Ecosystems

Introduction

The ocean is home to some of the most extraordinary and diverse creatures, each playing a crucial role in maintaining marine ecosystems. Among these fascinating organisms are tunicates, also known as sea squirts. Despite their seemingly simple appearance, tunicates belong to the phylum Chordata, making them close relatives of vertebrates, including humans. These marine invertebrates exhibit remarkable biological and ecological traits that have intrigued scientists for centuries.

What Are Tunicates?

Tunicates are marine animals that belong to the subphylum Tunicata (or Urochordata) within the phylum Chordata. They are named for their outer covering, known as the "tunic," which is a cellulose-like sac that protects their soft bodies. Unlike most invertebrates, tunicates share key characteristics with vertebrates, such as a notochord during their larval stage. This developmental feature places them in the same phylum as fish, amphibians, reptiles, birds, and mammals.

Classification and Diversity

Tunicates are classified into three main classes:

Ascidiacea (Sea Squirts): These sessile tunicates attach themselves to hard surfaces such as rocks, coral reefs, and ship hulls. They are the most diverse and widely studied group.

Thaliacea (Salps): Unlike sea squirts, salps are free-floating and often form long, chain-like colonies. They are highly efficient at filtering ocean water, playing a significant role in carbon cycling.

Appendicularia (Larvaceans): These tunicates retain their larval form throughout life and construct mucus houses to filter microscopic particles from the water.

Morphology and Anatomy

Tunicates have a unique body structure adapted to their marine environment. Their body is enclosed within a flexible tunic made of tunicin, a cellulose-like substance. They have two siphons: an incurrent siphon, which draws water in, and an excurrent siphon, which expels it after filtration. Inside the body, the pharyngeal basket filters plankton and organic particles from seawater, while the endostyle aids in mucus production, a key feature linked to vertebrate thyroid glands.

Life Cycle and Reproduction

Tunicates exhibit both sexual and asexual reproduction. Most sea squirts are hermaphroditic, producing both eggs and sperm. Fertilization occurs externally or internally, depending on the species. Larvae resemble tadpoles, possessing a notochord and a simple nerve cord, features that link them to vertebrates. After a brief free-swimming stage, the larvae settle onto a surface and metamorphose into their adult form, resorbing their tail and nerve cord. Salps and larvaceans, in contrast, remain free-swimming throughout their lives and can reproduce rapidly, forming dense populations in nutrient-rich waters.

Ecological Importance

Tunicates play vital roles in marine ecosystems:

Water Filtration: By filtering plankton and organic debris, they help maintain water quality and nutrient balance.

1. FERTILIZATION CLEAVAGE GASTRULATION Sky O. Lawa-an B.S Medical Biology TUNICATE

2. FERTILIZATION Egg Structure : Tunicate eggs are surrounded by a fibrous vitelline coat and follicle cells that produce sperm-attracting substances. Sperm Entry : Sperm passes through the follicle cells and binds to glycosides on the vitelline coat, leaving mitochondria behind as it enters the egg. Polyspermy Prevention : Rapid modification of the vitelline coat occurs to prevent polyspermy, ensuring only one sperm fertilizes the egg. Calcium Waves : After fertilization, free calcium ions are released in waves, prompting physiological and structural changes necessary for development. Ooplasmic Segregation : This process determines the embryo's dorsoventral and anteroposterior axes through cytoplasmic rearrangement.

3. Overall Reproduction Sexual Reproduction : Tunicates can reproduce sexually through broadcast spawning or philopatric methods. Broadcast spawners release eggs and sperm into the water, while philopatric species fertilize internally. Larval Development : Fertilized eggs develop into larvae, which either disperse widely or settle near the parent colony. Larvae undergo retrogressive metamorphosis, losing some larval structures and developing into sessile adults. Asexual Reproduction : Colonial tunicates reproduce asexually through budding, where new individuals develop from stolons or body parts. This allows colonies to grow and survive for decades. Self/Non-Self Recognition : Tunicates have mechanisms to recognize self from non-self, preventing unrelated colonies from fusing and parasitizing them.

4. CLEAVAGE C leavages in the ontogeny of the tunicate Oikopleura dioica . (Left) Projections of the upper (animal) half of DIC-stacks recorded for 4D microscopy. (Line drawings in Left) View from animal pole. (Line drawings in Right) View from vegetal pole. Asterisk marks the position of the blastopore. (Right) 3D representation of the positions of the nuclei of each cell in the 4D microscopy software as seen from the vegetal pole. Dashed line indicates axis of bilateral symmetry. Note bilateral symmetry in these early embryonic stages. Tunicates undergo holoblastic bilateral cleavage . This type of cleavage involves the entire egg being divided into smaller cells, or blastomeres, and establishes bilateral symmetry early in development.

5. GASTRULATION Tunicates primarily undergo invagination during gastrulation. This process involves the inward folding of a region of cells to form a cup-shaped structure, creating the archenteron, which will develop into the digestive tract. The blastopore, the opening of the invaginated region, eventually becomes the anus in tunicates. Invagination Process : The initial stage of gastrulation involves the invagination of the endodermal cells at the vegetal pole of the blastula. These cells move inward to form a pouch-like structure. Formation : This movement creates the archenteron, the primitive gut, which will develop into the digestive system. Involution Process : Involution involves the inward movement of mesodermal cells. These cells migrate and spread to occupy the space between the ectoderm and the invaginating endoderm. Formation : The mesodermal cells will give rise to various structures, including muscles and other internal organs. Ectodermal Spreading Process : Following invagination and involution, the ectodermal cells spread to cover the surface of the embryo. Function : This layer of cells will eventually form the epidermis and contribute to the development of the nervous system.

6. Figure 2.. Ciona gastrulation.  Confocal images of gastrulating Ciona robusta embryos at the stages indicated. The panels on the left are derived from confocal stacks of a phalloidin-stained embryo early in gastrulation, with xy , xz and yz sections shown. The surface-rendered view of the stack is from the vegetal side. The green asterisk marks the open blastopore/archenteron. Yellow asterisks mark notochord precursor cells. The panels on the right show an older embryo near the end of gastrulation/beginning of neurulation. The red dot indicates the neural groove just starting to form on the posterior dorsal side. Scale bars=50μm.

7. Figure 3.  Two-step model for endoderm invagination in ascidian development. Figure from (Sherrard et al., 2010). Tunicates primarily undergo invagination during gastrulation. This process involves the inward folding of a region of cells to form a cup-shaped structure, creating the archenteron, which will develop into the digestive tract. The blastopore, the opening of the invaginated region, eventually becomes the anus in tunicates.

8. Figure 4.  Blastophore of Salpa fusiformis. Image from (Sutton, 1960). The precise developmental age of the blastophore is not known. Abbreviations: I.F., Incubation fold; A., atrium; I.C., incubation cavity; C., calymnocyte ; B., blastomere; C.C., central cavity; N-C.I., neuro-chordal invagination; N-C.C. Neuro-chordal cells.

9. REFERENCES Ghiselin , M. T. (2024, October 10).  Tunicate | Anatomy, Habitat & Adaptations . Encyclopedia Britannica. https://www.britannica.com/animal/tunicate Winkley, K. M., Kourakis , M. J., DeTomaso , A. W., Veeman , M. T., & Smith, W. C. (2019). Tunicate gastrulation. Current Topics in Developmental Biology/Current Topics in Developmental Biology , 219–242. https://doi.org/10.1016/bs.ctdb.2019.09.001 https://www.researchgate.net/figure/First-cleavages-in-the-ontogeny-of-the-tunicate-Oikopleura-dioica-Left-Projections-of_fig15_292258324